Abstract
A lignin-based gel prepared by the chemical crosslinking of hardwood acetic acid lignin (AL) with poly(ethylene glycol) diglycidyl ether has been reported to shrink in water and organic solvents but swell specifically in aqueous binary solutions. In this study, the AL-based gel was also found to swell in lithium-salt-containing organic solvents, namely, liquid electrolytes. The uptake of salt solutions reached five times the dry weight of the gel. The ionic conductivity of the gel swollen with 1 M LiBF4 in propylene carbonate or a mixed solution (1:1, v/v) of ethylene carbonate and dimethyl carbonate exceeded 1 mS cm−1 at room temperature (25 °C), suggesting that this gel can be applied as a gel electrolyte for lithium-ion batteries (LIBs). A prototype LIB was assembled with the AL-based gel electrolyte and LiCoO2/graphite-based electrodes and exhibited low bulk and charge transfer resistances of 4.1 and 9.7 Ω, respectively. Moreover, its initial specific capacity reached 104 mAh g−1 at a current density of 28 mA g−1, which is comparable to that of a reference LIB assembled using a commercial polyethylene separator. These results indicate the significant potential of this lignin-based gel for application in energy storage devices.
1 Introduction
Lignin is the most abundant aromatic polymer in nature, and its utilization as a high-value-added material is in demand (Bajwa et al. 2019; Calvo-Flores and Dobado 2010; Duval and Lawoko 2014; Ragauskas et al. 2014). Various types of functional materials have been developed from lignin, such as carbon fibers for electronic devices (Pakkang et al. 2023; Qu et al. 2021), polyurethane foams for oil–water separation (Chen et al. 2023), and whitened nanoparticles for adhesives and fillers (Shikinaka and Otsuka 2022). Currently, lignin-based gels have also attracted much attention (Dominguez-Robles et al. 2018; Rico-Garcia et al. 2020; You et al. 2020), because of their wide range of functions and applications, such as self-healing, conductivity, and drug delivery (Khattab and Kamel 2022; Kim et al. 2019; Li et al. 2020).
Polymer gels are soft and wet materials capable of undergoing large deformations due to the incorporation of solvents into their crosslinked polymer networks. Swelling and shrinking are important properties of gels that are responsive to external stimuli, such as temperature, pH, solvent composition, and salt concentration (Echeverria et al. 2018; Zhang and Khademhosseini 2017). In particular, salt effects have attracted fundamental research interest. Basically, gel swelling is promoted by the high affinity between the network polymer and solvent and the good flexibility of the network polymer, but is inhibited by the high external osmotic pressure induced by salt addition (Quesada-Pérez et al. 2011; Zhang et al. 1997). However, several hydrogels and organogels have been reported to swell in salt-containing solutions, such as a polyacrylamide (PAA)-based hydrogel with ionized amide groups (Ohmine and Tanaka 1982), poly(ethylene glycol) (PEG)-based organogels with sulfonate and amino groups (Matsumoto and Endo 2010), and PAN-based organogels with nitrile groups (Verdier et al. 2019). The specific swelling behaviors of these gels were attributed to the coordination of cations in salts to each functional group in the network polymer. The swelling of these gels in salt-containing solutions is of great significance in terms of practical applications because such gels can acquire new functions and applications based on the salt, for example, conductivity for a gel electrolyte, freeze resistance for structural material in cold region (Morelle et al. 2018), and nutrient retention for a fertilizer (Tubert et al. 2018). So far, few studies have evaluated lignin-based gels soaked in salt-containing solutions as gel electrolytes, and the swelling behavior itself has not been mentioned (de Haro et al. 2021; Gong et al. 2016; Trano et al. 2022).
In this study, the swelling of a lignin-based gel in organic solvents was investigated in the presence and absence of salt. The gel was prepared by chemical crosslinking of hardwood acetic acid lignin (AL) with poly(ethylene glycol) diglycidyl ether (Nishida et al. 2003); called “AL-P gel” in this article. The composition of AL and PEG moieties in the AL-P gel was 41 and 59 wt%, respectively, which was determined by a previously reported method (Morgan 1946; Siggia et al. 1958). Interestingly, this AL-P gel has been reported to shrink in water and organic solvents but swell specifically in aqueous organic solvents because of the amphiphilic nature of the AL-P gel (Nishida et al. 2003; Taira et al. 2021) As the AL-P gel retains some phenolic hydroxy groups that can coordinate to cations, an organogel derived from AL-P is expected to swell in response to salt addition. This hypothesis was preliminarily verified by swelling tests conducted on AL-P gel in organic solvents containing lithium salts. Therefore, the objective of this study was to investigate the swelling properties of AL-P gels by changing the crosslinker lengths and solution compositions, that is, single- or multi-organic solvent systems with or without lithium salts. Furthermore, the electrochemical properties of the swollen AL-P gel were characterized as a gel electrolyte with dual functions as a separator and liquid electrolyte, that can suppress the leakage, for lithium-ion batteries (LIBs).
2 Materials and methods
2.1 Materials
Acetic acid (AcOH), poly(vinylidene difluoride) (PVDF), N-methyl-2-pyrrolidone (NMP), acetylene carbon black (50 % compressed), graphite (powder), lithium cobalt oxide (LiCoO2), and lithium tetrafluoroborate (LiBF4, battery grade) were purchased from Fujifilm Wako Chemicals Co., Ltd. (Osaka, Japan). Poly(ethylene glycol) diglycidyl ether (PEGDGE) with 13 and 22 polyethylene oxide (PEO) units (Denacol EX-841 and EX-861, respectively) was kindly provided by Nagase ChemteX Co. (Osaka, Japan). Propylene carbonate (PC) and a mixed solution (1:1, v/v) of ethylene carbonate (EC) and dimethyl carbonate (DMC) in electrochemical grade were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). All chemicals were used as received.
2.2 Preparation of AL-based gels and reference gels
2.2.1 Purification of AL
AL was produced by Guangzhou Yinnovator Biotech Co. (Guangzhou, China) from a black liquor of atmospheric AcOH-pulping of eucalyptus (Uraki et al. 1991, 1995). The AL was dissolved in AcOH at room temperature (RT, 25 °C), and the AcOH-insoluble fraction was removed by filtration. The filtrate was then poured into water to precipitate the AcOH-soluble fraction of AL, which was then filtered, washed with water, and freeze-dried to yield purified AL (see Supporting Information for the molar mass and hydroxy (OH) content in Supplementary Figures S1 and S2, respectively).
2.2.2 Preparation of thick gel films
Purified AL (5 g) was dissolved in a 3.3 M NaOH aqueous solution (15 mL) with stirring at RT for 1 d. PEGDGE with PEO units of 13 or 22 (3 or 4 g, respectively) was added to the solution and stirred at RT for 5 min. The resultant mixture was transferred into a glass Petri dish with a diameter of 90 mm and heated in an oven at 80 °C for 1 h to yield an AL-based gel. The gel was cut into circular shapes with a diameter of 16 mm using a cork borer. The resulting gel was neutralized with AcOH, repeatedly washed with distilled water, and freeze-dried. The obtained gel is abbreviated as AL-P13 or AL-P22 gel, corresponding to the PEO units of PEGDGE used.
Reference gels were prepared using only crosslinking reagents. PEGDGE with PEO units of 13 or 22 (7 g or 10 g, respectively) in distilled water (2 mL) was added to a 10 M KOH aqueous solution (2 mL) and stirred at RT for 5 min. The solution was transferred into a glass Petri dish with a diameter of 90 mm and heated in an oven at 85 °C for 2 h to yield a PEGDGE-based gel. The resultant gel was cut off, neutralized, and freeze-dried in the similar manner to that for the AL-P gels. The obtained gel is abbreviated as P13 or P22 gel. The thick gel films prepared above were used for solution uptake measurements and thermal analysis.
2.2.3 Preparation of thin gel films
For electrochemical analysis, a thin gel film of AL-P22 gel was prepared as follows: A mixture of AL and PEGDGE with 22 PEO units in an NaOH aqueous solution was prepared as mentioned above. After preheating at 70 °C for 10 min, the mixture was cast onto a glass plate using a doctor blade with a clearance of 200 μm and then left in air at RT for overnight to generate a cast film of the AL-P22 gel. After cutting off, neutralizing, and freeze-drying, the prepared thin gel film was immersed overnight in 1 M LiBF4/PC or 1 M LiBF4/EC-DMC at RT and subjected to electrochemical analysis.
2.3 LIB assembly
2.3.1 Preparation of cathode and anode
For the impedance and charging/discharging measurements, an LIB prototype was assembled with the thin film of AL-P22 gel swollen with a liquid electrolyte, cathode, and anode. First, the cathode was prepared as follows: LiCoO2, carbon black, and PVDF (90:5:5, w/w) were dispersed in NMP under stirring at 2000 rpm for 15 min using a planetary centrifugal mixer (AR-100, THNKY Co., Tokyo, Japan), and the resulting slurry was cast onto an aluminum plate using a doctor blade with a clearance of 200 μm, followed by vacuum drying overnight at 80 °C and compressing at 3 MPa for 30 s. The resulting material was cut into a circular shape with a diameter of 16 mm to yield the cathode. The anode was prepared from a slurry of graphite and PVDF (95:5, w/w) in NMP by casting it onto a copper plate.
2.3.2 Assembly of LIB prototypes
An LIB prototype was assembled using a two-electrode-type cell (2E-CELL-SUS, Eager Co., Osaka, Japan) in a glove box filled with Ar gas. First, the anode was placed at the bottom of the measurement cell. The AL-P22 gel swollen in 1 M LiBF4/EC-DMC, prepared as described in Section 2.2.3, was placed on the anode in the cell, followed by the cathode, to obtain an AL-P22 gel electrolyte-based LIB prototype. For comparison, a reference LIB prototype was assembled using a commercial separator based on a porous polyethylene (PE) membrane soaked overnight in 1 M LiBF4/EC-DMC at RT.
2.4 Solution uptake measurements
The thick gel film prepared as described in Section 2.2.2 was soaked for 24 h at RT in PC or 1:1 (v/v) EC-DMC containing different concentrations of LiBF4 (0, 1, and 2 M). After removing excess solution using a Kimwipe, the mass change was measured by weighing the gel films before and after soaking. The solution uptake by the gel was calculated using the following equation:
where Wdry is the initial dry weight (g) of the gel film and Wswell is the weight (g) after soaking in the solution.
2.5 Thermal analyses
2.5.1 Thermogravimetric analysis (TGA)
TGA was performed using a TG-DTA-2000S instrument (MAC Science, Ltd.). Approximately 5–10 mg of the dried sample was placed in an aluminum pan and subjected to the TG measurement in the temperature range from RT to 500 °C at a heating rate of 10 °C min−1 under a N2 flow at 150 mL min−1. The thermal decomposition temperature (Td) was determined as the onset of significant weight loss (≥5 %) of the sample during heating (Td−5 %).
2.5.2 Thermomechanical analysis (TMA)
TMA was performed under compression mode using a TMA-4000S (MAC Science, Ltd.), according to a previously reported method (Kubo et al. 1996). The thick film of the AL-P22 or P22 gel immersed in a 1 M LiBF4/PC solution, prepared as described in Section 2.2.2, was cut using a cork borer into a circular shape with a diameter of 5 mm and then placed in an aluminum pan. After covering with an aluminum plate, the prepared pan was placed on the support tube and compressed by a quartz probe at a 5 gf-load during heating from RT to 300 °C at a heating rate of 5 °C min−1 under a N2 flow at 150 mL min−1. The relative thickness (L/L 0 ) of the gel sample was plotted against temperature, where L 0 and L are the initial thickness and the thickness of the sample during the TMA measurement, respectively.
2.6 Electrochemical analyses
2.6.1 Ionic conductivity of gels
The thin film of AL-P22 gel or the thick film of P22 gel swollen with 1 M LiBF4/PC or 1 M LiBF4/EC-DMC, prepared as described in Sections 2.2.3 and 2.2.2, respectively, was sandwiched between two stainless-steel (SS) plates in a glove box filled with Ar gas. The plates were then inserted into a two-electrode type measurement cell (2E-CELL-SUS, Eager Co., Osaka, Japan). Electrochemical impedance spectroscopy (EIS) measurements were performed at RT using an AUTOLAB potentiostat/galvanostat instrument (Autolab PGSTAT302N FRA32M, Metrohm Autolab B.V., Tokyo, Japan) with an amplitude of 10 mV and a frequency range of 1 Hz to 1 MHz to obtain Nyquist plots. The bulk resistance (R) of each gel electrolyte was calculated from the intercept of the x-axis in the Nyquist plots, and the ionic conductivity (σ) was calculated using the following equation:
where σ is the ionic conductivity (S cm−1); R is the measured bulk resistance (Ω); S is the contact area between the gel film and SS plates (cm2); and l is the thickness of the gel film (cm), measured using a caliper. The bulk resistance was not corrected for the resistance of the SS plates because it was negligible or almost zero.
2.6.2 Impedance of LIB prototypes
The bulk and charge transfer resistances of the assembled LIBs were measured using EIS. Bulk resistance was calculated from the intercept of the x-axis, whereas charge transfer resistance was calculated from the diameter of the semicircle in the Nyquist plots (Yang and Rogach 2019).
2.6.3 Charging/discharging capacities of LIB prototypes
The galvanostatic charge/discharge (GCD) measurement for the assembled LIBs was carried out at a current density of 28 mA g−1 in a voltage range from 3.0 to 4.2 V. This applied current density was determined to complete the charging ideally in 5 h because 140 mAh g−1/5 h = 28 mA g−1, where 140 mAh g−1 is the theoretical capacity of the LiCoO2 cathode (Zhang et al. 2021; Aoxia and Sen 2016). The GCD test was repeated three times.
3 Results and discussion
3.1 Unique swelling properties of AL-P gels in lithium salt-containing organic solvents
AL-P13 and AL-P22 gels were prepared by crosslinking AL using PEGDGE with 13 and 22 PEO units, respectively (Nishida et al. 2003; Taira et al. 2021). As reference gels, the P13 and P22 gels were synthesized only from PEGDGE. However, the preparation conditions required higher PEGDGE concentrations and gelation temperatures than those required for the AL-P gels, suggesting that AL molecules act as effective crosslinking points for gelation with PEGDGE.
To quantitatively investigate the swelling capability of the gels, solution uptake (w/w) was measured at RT. As shown in Figure 1a, the uptakes of the P13 and P22 gels for neat PC were 5.5 and 6.5, respectively. As demonstrated in Figure 2, such high uptake was accompanied by apparent gel expansion, indicating the good swelling behavior of the reference gel with pure organic solvent. When LiBF4 salt was added to PC at 1 M, the solution uptake increased to 7.7 and 9.2, which corresponds to a 1.4-fold uptake for neat PC. However, the solution uptake at 2 M LiBF4 concentration decreased compared to that at 1 M. By contrast, when the gels were immersed in EC-DMC, the solution uptake increased with an increase in the LiBF4 concentration up to 2 M. The average uptakes of the P13 and P22 gels for 2 M LiBF4/EC-DMC were 7.3 and 8.2, which correspond to 1.4- and 1.3-fold uptakes for neat EC-DMC, respectively. Such small increases in the solution uptake of the P13 and P22 gels with the addition of LiBF4 suggest their weak responsiveness to lithium salts, regardless of the type of organic solvent.
Swelling properties of (a) reference gels prepared using only poly(ethylene glycol) diglycidyl ether (PEGDGE) with polyethylene oxide (PEO) units of 13 and 22 (P13 and P22 gels, respectively) and (b) hardwood acetic acid lignin (AL)-based gels crosslinked using PEGDGE with PEO units of 13 and 22 (AL-P13 and AL-P22 gels, respectively) in propylene carbonate (PC), a mixed solution (1:1, v/v) of ethylene carbonate (EC) and dimethyl carbonate (DMC), and their salt-containing solutions with 1 and 2 M LiBF4.
Images of AL-P22 gel and P22 gel in the dry and wet states in PC and 1 M LiBF4/PC solutions.
As shown in Figure 1b, the AL-P13 and AL-P22 gels showed much larger increments in solution uptake in response to the LiBF4 addition than the P13 and P22 gels, although the maximum uptake of the AL-P gels was lower than that of the reference gels. The solution uptakes of the AL-P22 gel in 1 M and 2 M LiBF4 solutions in PC were 5.0 and 5.3, respectively, which corresponded to 3.0- and 3.2-fold uptakes for the neat PC. Similarly, the AL-P22 gel showed 2.8- and 3.0-fold uptakes for 1 M and 2 M LiBF4 solutions in EC-DMC, respectively, compared to those for neat EC-DMC. In contrast, the AL-P13 gel showed the maximum solution uptake for 1 M LiBF4/PC and EC-DMC (2.8- and 3.1-fold for the neat organic solvents, respectively), although the uptake slightly decreased at 2 M LiBF4. These remarkable increases in the solution uptake of the AL-P gels for LiBF4-containing organic solvents must be caused by the AL moiety in the gels, particularly the phenolic OH groups that can coordinate with the lithium cations. In addition, the AL-P22 gel exhibited the larger uptake of the LiBF4-containing solutions than the AL-P13 gel (Figure 1b). As shown in Figure 1a, the P22 gel adsorbed more organic solvents than the P13 gel. This result suggests that the solvent uptake was facilitated by the longer PEO unit of the crosslinker used, probably due to the better extensibility of the PEO moiety in the gels. Based on these results, the longer PEO moiety in the AL-P22 gel should also contribute to the higher salt-solution uptake than that of the AL-P13 gel.
3.2 Ionic conductivity of AL-P22 and P22 gels swollen in 1 M LiBF4/PC or EC-DMC
High ionic conductivity is one of the promising functions of salt-incorporated gels (Cheng et al. 2018; Sekhon 2003). To investigate this function of the AL-P22 gel using EIS measurements, a thin film was prepared via the casting technique, and the ionic conductivity of the gel swollen in a 1 M LiBF4 solution was calculated from the obtained bulk resistance. As shown in Table 1, the ionic conductivity of the AL-P22 gel was 1.0–1.1 mS cm−1, which did not depend on the type of organic solvents used. On the other hand, the P22 gel as a reference showed slightly higher ionic conductivity than the AL-P22 gel, probably because of the larger solution uptakes of the P22 gel (Figure 1) (Michot et al. 2000). In general, the ionic conductivity required for the gel electrolyte is 1 mS cm−1 (Choi et al. 2011; Li et al. 2008). The ionic conductivities of the thin AL-P22 gel films swollen in 1 M LiBF4/PC and EC-DMC meet this criterion, suggesting their potential application as gel electrolytes.
Ionic conductivity of AL-P22 gel and P22 gel.
| Gel | Solution | l/mm | R/Ω | S/cm2 | δ/mS cm−1 |
|---|---|---|---|---|---|
| AL-P22 | 1 M LiBF4/PC | 0.058 | 2.7 | 2.0 | 1.1 |
| 1 M LiBF4/EC-DMC | 0.077 | 3.8 | 2.0 | 1.0 | |
| P22 | 1 M LiBF4/PC | 0.58 | 23 | 1.6 | 1.6 |
| 1 M LiBF4/EC-DMC | 1.4 | 44 | 2.0 | 1.6 |
-
Where l is the thickness of each gel film; R is the bulk resistance; S is the contact area between each gel film and SS plates; and δ is the ionic conductivity of each gel.
3.3 Thermal properties of AL-P22 and P22 gels in the dry and wet states
TGA and TMA measurements were performed to evaluate the thermal stability of the AL-P22 gel, which is also an important property to be used as a gel electrolyte (Chen et al. 2022; Cheng et al. 2018). The thermal degradation temperatures, Td−5 %, of the AL-P22 and P22 gels in the dry state were almost similar at 310 °C and 326 °C, respectively, as shown in their TGA profiles (Figure 3a). Moreover, the AL-P22 gel showed a higher Td−5 % than the original AL powder (278 °C), suggesting that the thermal stability of AL was successfully improved by crosslinking with PEGDGE.
Thermal properties of gels: (a) comparison of the thermogravimetric (TG) curves of AL-P22 gel and P22 gel in the dry state with those of purified AL powder and (b) the profiles of thermomechanical analysis (TMA) of AL-P22 gel and P22 gel in the swollen state with 1 M LiBF4 in 1:1 (v/v) EC-DMC.
When the P22 gel was further heated above Td−5 %, the residual weight reached almost zero at approximately 400 °C, indicating that complete pyrolysis had occurred. However, in the case of the AL-P22 gel, 37 wt% of the residue remained even after heating to 500 °C, plausibly because of the high thermal stability of the aromatic rings of the AL moiety. This result suggests superior flame retardancy of the AL-P22 gel compared to that of the reference P22 gel.
The dimensional stability under thermal compression of the gels swollen in 1 M LiBF4/EC-DMC was investigated using TMA. As shown in Figure 3b, the TMA profile of the P22 gel was flat below 100 °C, indicating that no heat deformation occurred. However, it discontinuously deformed with increasing temperature, and a drastic decrease in its volume was observed at 200 °C. This result suggests the collapse of the shape of the P22 gel. On the other hand, AL-P22 gel was gradually compressed with increasing temperature, and no drastic thermal deformation occurred until 300 °C. This result demonstrates that the AL-P22 gel did not collapse upon heating at such high temperatures, indicating its superior safety as a gel electrolyte to the reference gel.
3.4 Electrochemical performance of LIB prototype assembled with AL-P22 gel electrolyte
Typically, LIBs consist mainly of a cathode, anode, liquid electrolyte, and separator. The gel electrolyte functions as both a separator and a liquid electrolyte in the LIB, suppressing the undesirable leakage of the liquid electrolyte from the LIB cell. In this study, a prototype LIB was assembled with a LiCoO2-based cathode, graphite-based anode, and thin film of AL-P22 gel swollen in 1 M LiBF4/EC-DMC as the gel electrolyte. On the other hand, the cell could not be assembled using P22 gel because a self-standing thin film (thickness: <0.5 mm) was difficult to prepare via a casting technique similar to that used for the AL-P22 gel. Moreover, when soaked in liquid electrolytes, the barely fabricated thin film of the P22 gel was too sticky to handle for LIB cell assembly. Thereby, a reference LIB prototype was prepared using a commercial separator based on a porous polyethylene (PE) membrane.
LIBs assembled with the AL-P22 gel and PE membrane were first subjected to EIS measurements to investigate the two types of resistance in the cells. The corresponding Nyquist plots are shown in Figure 4. The bulk resistance reflects the ion mobility in the gel electrolyte or liquid electrolyte (Abe et al. 2019), whereas the charge transfer resistance reflects the ion mobility at the interface between the electrolyte and electrodes (Yamada et al. 2006). The LIB with the AL-P22 gel exhibited a bulk resistance of 4.1 Ω and a charge transfer resistance of 9.7 Ω. Both values are comparable to those of the reference LIB (3.6 and 14 Ω, respectively). These results suggest that the Li-ion mobilities of the two LIBs were almost identical.
Nyquist plots of lithium-ion battery (LIB) prototypes assembled with LiCoO2/graphite-based electrodes and AL-P22 gel electrolyte or a commercial polyethylene (PE) separator. 1 M LiBF4 in 1:1 (v/v) EC-DMC was used as the liquid electrolyte for both LIB assemblies.
The charging and discharging performances of the LIBs assembled with the AL-P22 gel and PE membrane were investigated using GCD measurements. Figure 5 shows the charge and discharge profiles of the 1st cycle. The discharge capacity of the LIB with the AL-P22 gel was 104 mAh g−1, which was 93 % that of the reference LIB with the PE membrane (112 mAh g−1). This result demonstrates the good performance of the LIB using the AL-P22 gel electrolyte, which can be attributed to the high liquid electrolyte uptake of the AL-P22 gel and the resultant high ion conductivity (Table 1).
Charge and discharge voltage profiles of first cycle of the LIB prototypes assembled with AL-P22 gel electrolyte or PE separator using 1 M LiBF4 in 1:1 (v/v) EC-DMC as the liquid electrolyte. This measurement was performed from 3.0 to 4.2 V at 28 mAg−1.
Unfortunately, the discharge capacities of both LIBs assembled with the AL-P22 gel and commercial PE membrane gradually decreased from the 2nd cycle onward (Supplementary Figure S3). Such early degradation of performance must be caused by the contamination of by a small amount of water in the measurement cells (Pakkang et al. 2023), because the humidity in the globe box used for the LIB assembly could not be completely removed.
4 Conclusions
This study investigated the specific swelling behavior of AL-based gels in response to the addition of lithium salts. AL-P13 and AL-P22 gels were prepared by chemically crosslinking AL using PEGDGE with PEO units of 13 and 22, respectively. Both gels hardly swelled in neat PC and EC-DMC; however, when 1 M LiBF4 was added to the respective organic solvent systems, the solution uptake of both AL-P gels significantly increased by 3-fold. From a comparison of the swelling behaviors of the P13 and P22 gels, the lithium salt-responsive swelling ability of the AL-P gels was attributed to their AL moiety, which can coordinate with the lithium cation. The AL-P22 gel swollen in 1 M LiBF4/EC-DMC showed an ion conductivity of over 10−3 S cm−1, which is adequate for practical use as a gel electrolyte for LIB. Moreover, thermal analysis revealed that the AL-P22 gel swollen in 1 M LiBF4/EC-DMC did not collapse up to 300 °C, indicating its superior safety as a gel electrolyte compared with that of the reference P22 gel. A LIB prototype assembled with the AL-P22 gel electrolyte exhibited a low charge transfer resistance and high initial specific capacity of 104 mA g−1 compared to those of a reference LIB assembled with a commercial PE separator. These results demonstrate the significant potential of the AL-P22 gel swollen in organic solvents containing lithium salts for application in energy storage devices. The properties of gel electrolytes based on other lignin preparations, such as milled wood lignin and kraft lignin, are currently under investigations.
Abbreviations
- AcOH
-
Acetic acid
- AL
-
Acetic acid lignin
- DMC
-
Dimethyl carbonate
- EIS
-
Electrochemical impedance spectroscopy
- EC
-
Ethylene carbonate
- GCD
-
Galvanostatic charge/discharge
- LIB
-
Lithium-ion battery
- NMP
-
N-methyl-2-pyrrolidone
- PAA
-
Polyacrylamide
- PE
-
Polyethylene
- PEGDGE
-
Poly(ethylene glycol) diglycidyl ether
- PEO
-
Polyethylene oxide
- PC
-
Propylene carbonate
- PVDF
-
Poly(vinylidene difluoride)
- RT
-
Room temperature
- SS
-
Stainless-steel
Funding source: Japan Society for the Promotion of Science
Award Identifier / Grant number: 21H02007
Award Identifier / Grant number: JP20J20415
-
Author contributions: ST conceived and planned the experiments. FH carried out all the experiments, and FH and ST wrote the original manuscript. SS revised and edited the manuscript. SS, KS, KS, and YU supervised the project. All authors read and approved the final manuscript.
-
Competing interests: The authors declare no competing financial interests.
-
Research funding: This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI (grant numbers: JP20J20415 and 21H02007).
-
Data availability: The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
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Supplementary Material
This article contains supplementary material (https://doi.org/10.1515/hf-2023-0067).
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